A conventional hybrid vehicle is comprised of an internal combustion engine and an electric motor having a generating function, and a power of the engine is distributed to the motor side and an output side connected to drive wheels through a power distribution device as a differential mechanism. The power distribution device is adapted to perform a differential action among rotary members individually connected to the engine, the motor, and an output member. Specifically, the power distribution device distributes power from the engine to the motor side and the output side, and an engine speed is controlled by a reaction force of the motor serving as a generator. In order to convert an electric power generated by the motor into a mechanical power, a second motor is connected to the output member or an output shaft. The hybrid vehicle thus structured is called “two-motor type hybrid vehicle”, and in the hybrid vehicle of this kind, the engine and the motor can be operated separately. In the hybrid vehicle, therefore, the drive modes can be selected from engine mode where the vehicle is powered only by the engine, motor mode where the vehicle is powered only by the motor, and hybrid mode where the vehicle is powered by both engine and motor.
For example, US 2009/0082171 A1 describes a hybrid vehicle comprised of an engine and two motors. The hybrid vehicle taught by US 2009/0082171 A1 is provided with a planetary gear set including a ring gear being selectively connectible to the first motor/generator and the engine through a clutch or brake, a sun gear being continuously connected to the second motor/generator, and a carrier member continuously connected to drive wheels through an output member. According to the teachings of US 2009/0082171 A1, the transmission includes clutches and brakes that are selectively engageable alone or in different combinations to connect different ones of the nodes to one another or to a stationary member, thereby establishing the various operating modes. Specifically, the operation mode is shifted among the two motor electric-only mode where both motor/generators are operated, the one motor electric-only mode where only the second motor/generator is operated, and the output split mode where the engine and the second motor/generator are operated.
Another example of the hybrid vehicle is described in Japanese Patent Laid-Open No. 08-295140. According to the teachings of Japanese Patent Laid-Open No. 08-295140, a planetary gear unit serves as the power distribution device in which a carrier is connected to an engine, a sun gear is connected to a generator/motor, and a torque of an electric motor is added to a torque of a ring gear. To this end, both the engine and the carrier are stopped by the brake. The power distribution device is used as a speed reducer by stopping the carrier by the brake when the vehicle is powered by the generator/motor while stopping the engine.
In order to reduce fuel consumption, in the hybrid vehicle, the engine is operated in an optimally fuel efficient manner while regenerating energy. To this end, an EV mode is established by supplying power to the motor under a condition where a state of charge of a battery is sufficient and an accelerator is not opened widely. In this situation, the engine is stopped to reduce the fuel consumption while being disconnected from a powertrain to the drive wheels thereby reducing power loss resulting from rotating the engine passively.
If the state of charge of the battery connected to the motor becomes low during the EV mode, the engine has to be restarted to ensure the drive force and to charge the battery. In addition, the engine is also needed to be started when an accelerator pedal is depressed deeply to generate a large drive force. In those cases, a motoring torque for cranking the engine is generated by the motor serving as a power source. As described, according to the teachings of the foregoing prior art documents, both motor/generators are used as motors under the EV mode, and any one of the motor/generators is used to crank the engine. However, if one of the motor/generators is immediately switched to crank the engine upon satisfaction of restarting condition of the engine due to low battery condition under the EV mode where both motor/generators serve as motors, total drive force drops immediately to the drive force established by the other motor/generator. As a result of such sudden reduction in the drive force, shocks would be caused unintentionally. In addition, in order to immediately switch one of the motor/generators to crank the engine upon satisfaction of restarting condition while generating the drive force by the other motor/generator, a complex control is required and a control delay would be caused when restarting the engine.